Use `Either` instead of `Option` in `RuntimeValue`

[konnov]

RuntimeValue returns plenty of none() values, e.g.:

https://github.com/informalsystems/quint/blob/be8bb62dbc046da134d07f44367c4a60970adb45/quint/src/runtime/impl/runtimeValue.ts#L1330-L1354

We are losing an opportunity to explain what kind of error has happened. If we switch to Either, we can provide better feedback in the simulator/REPL.